1. Field of the Invention
The present invention relates to a cold-cathode fluorescent lamp for illuminating a liquid crystal display panel with illuminating light and a liquid crystal display provided with a backlight unit including the cold-cathode fluorescent lamp. More particularly, the present invention relates to the construction of a cold-cathode fluorescent lamp.
2. Description of the Related Art
Discharge tubes are used widely as low-power consumption light sources, high-luminance light sources, long-life light sources or small light sources in various illuminating devices. A low-pressure discharge tube formed by coating the inside surface of a mantle tube made of a transparent, insulating material, such as glass, with a fluorescent material, and sealing an inert gas and mercury in the mantle tube is known as a fluorescent lamp. Low-pressure discharge tubes of this kind are classified into those of a hot cathode type using electrons emitted by a hot cathode and those of a cold-cathode type using electrons emitted by a cold cathode.
An image display provided with a nonluminescent liquid crystal display panel visualizes electronic latent images formed on the liquid crystal display panel by an external illuminating system. The external illuminating system, excluding those using natural light, includes an illuminating device disposed opposite to the front or the back surface of the liquid crystal display panel. Most display devices required to have high luminance, in particular, are provided with an illuminating device disposed opposite to the back surface of the liquid crystal display panel. This illuminating device is called a backlight unit.
Backlight units are classified roughly into those of a direct type and those of a side-edge type. The side-edge type backlight unit has a linear light source, typically, a cold-cathode fluorescent lamp, extended along a side edge of a transparent light guide plate. The side-edge type backlight unit is used prevalently in thin display devices for personal computers and the like. Most large liquid crystal displays to be used as displays for display monitors and television receivers employ the direct backlight unit. The direct backlight unit includes an illuminating device disposed directly below the back surface of a liquid crystal display.
For example, a cold-cathode fluorescent lamp (CCFL) that energizes a fluorescent coating with electrons emitted by a cold cathode is used as a light source for the illuminating device of a liquid crystal display. In general, a cold electrode for emitting electrons is made of a metal, such as nickel. Such a cold electrode has a spurring property and hence the cold electrode wears out. Therefore, the electrode of the cold-cathode fluorescent lamp needs to be formed in a large size provided that the electrode can be received in a selected mantle tube. The large electrode has a large surface area. Consequently, current density per unit area decreases and the decreased current density reduces the amount of the material of the electrode deposited by sputtering on the inside surface of the mantle tube during an aging discharge process to which the cold-cathode fluorescent lamp is subjected prior to shipping.
The cold-cathode fluorescent lamp of this kind is required to shorten a discharge start time, i.e., a time needed to make the cold-cathode fluorescent lamp start an electric discharge. Particularly, it has been a problem that the discharge start time increases in darkness. It is known that, when part of the electrode made of nickel is deposited in a sputtered film on the inside surface of the mantle tube by sputtering, usually the discharge start time is shortened by electrons induced from the sputtered film. If a cup-shaped electrode having an open end having a small outside diameter as compared with the inside diameter of the mantle tube, deposition of the material of the cup-shaped electrode by sputtering on the inside surface of the mantle tube is promoted. However, the sputtering causes the electrode to wear out and shortens the life of the electrode. Moreover, the effect of the sputtered film formed by the aging process on shortening the discharge start time is not satisfactory.
Placing a discharge inducing substance in the mantle tube, in particular, in a part of the inside surface of the mantle tube near the electrode is effective to shorten the discharge start time. A known first discharge time shortening method deposits a metal compound film of a highly electron emissive metal compound on a surface of an electrode. For example, the highly electron emissive metal compound is a cesium compound, such as cesium chromate. A known second discharge time shortening method uses a mixture of a highly electron emissive metal compound, such as a cesium compound, and a mercury emissive substance and diffuses the highly electron emissive metal compound, such as the cesium compound, in a tube during mercury emission heating in a manufacturing process. A known third discharge time shortening method supplies a high current between about 8 and about 15 mA to an electrode having an outside diameter of 1.7 mm to deposit a sputtered film of the material of the electrode on a part near each of the electrodes of the inside surface of a mantle tube by heating the electrode. A known fourth discharge time shortening method uses a fluorescent film containing alumina.
More concretely, a discharge time shortening method disclosed in Japanese Patent Laid-Open No. 2001-76617 deposits a sputtered film containing, as a principal component, a metal or a metal compound on a part near a cup-shaped electrode of the inside surface of a mantle tube by sputtering, and induces an electric discharge by electrons emitted by the cup-shaped electrode when a voltage is applied to the cup-shaped electrode to start an electric discharge in a short time. A discharge time shortening method disclosed in Japanese Patent Laid-Open No. 2002-231133 uses a cup-shaped electrode having a large surface area, deposits a thin film containing a metallic material on the inside surface of the cup-shaped electrode, and forms a film of the metallic material on a part near each of the cup-shaped electrodes of the inside surface of a mantle tube by hot vaporization in an aging process.